The present invention relates to the die casting of aluminum alloys, and particularly to aluminum alloys used in hot chamber die casting machines.
Hot chamber type die casting machines include a container for molten metal which is installed adjacent the die casting machine. At least a portion of an injection pump is immersed in the molten metal in the container so that a plunger of the pump may draw the molten metal into the casting machine. For many years, this type of device has been used extensively for casting low melting point metals such as lead, tin and zinc. However, when used for relatively high melting point alloys such as aluminum, hot chamber die casting machines have proved unsatisfactory due to the corrosive effects of the molten alloys, which are very active chemically at high temperatures. In addition to causing deterioration of the high strength steel used to make the casting machine, the corrosion causes contamination of the composition of the cast products.
One conventional solution to this problem has been to use a so-called cold chamber casting machine, in which the molten metal is ladled into an unheated injection cylinder before each filling of the die. The main disadvantages of cold chamber die casting include the fact that when the molten metal is ladled into the casting chamber, a certain amount of oxide is simultaneously transferred as well. Also, it is difficult to determine the exact quantity of molten metal ladled, and further oxidation of the molten metal occurs during the filling of the injection cylinder, which reduces the quality of the molded parts.
For the above reasons, hot chamber casting is preferred because it is relatively faster and provides more uniform results than cold chamber casting, despite the fact that hot chamber casting is more complicated. As such, there have been many attempts over the years to adapt hot chamber casting machines to the corrosive effects of molten aluminum and other relatively high melting point alloys. These attempts typically approached the problem by protecting the metal of the hot chamber machine through ceramic or alloy coatings for portions of the machine coming in contact with the molten aluminum. Such attempts are described in U.S. Pat. Nos. 3,586,095; 4,091,970; 4,556,098; and 5,476,134, all of which are incorporated by reference.
None of these attempts have been particularly successful over the working life of a die casting machine, and as such, until the present invention, there has been little commercialization of hot chamber die casting machines for casting aluminum. As a result, designers of cast or molded parts often select plastic over aluminum due to its castability or moldability in a more efficient manner than cold chamber casting.
Thus, there is a need for a commercially acceptable way to cast aluminum parts using a hot chamber die casting machine. The present invention approaches the problem in a novel way, by passivating the aluminum alloy, or making it noncorrosive to the steel of the die casting machine.
Accordingly, it is a primary object of the present invention to provide a process for improving the productivity of aluminum castings from a hot chamber die casting machine.
Another object of the present invention is to provide a process for increasing the life span of an injection chamber for use in a hot chamber aluminum die casting machine.